The objective of this project is to examine the functions and mechanisms of action of membrane immunoglobulin (Ig) and helper T cells (Th) in the stimulation of murine B lymphocytes, using a combination of in vitro and in vivo analyses. 1. Antigen presentation and signal transduction by transfected Ig in vitro Membrane IgM molecules, which are specific for phosphorylcholine (PC) and contain normal heavy chains or mutations/deletions in transmembrane and cytoplasmic domains, have been expressed in mouse B lymphoma lines by transfection. These stable transfectants will be used to: i) define the pathways of antigen endocytosis and processing, ii) identify Ig-associated proteins, and iii) correlated these with patterns and efficiency of antigen presentation and signal transduction. 2. Functions of membrane Ig in transgenic B cells Transgenic mice expressing the wild-type and defective mutant PC- specific membrane IgM's will be produced. B cells from these mice will be examined for; i) activation by antigen and Th or anti-Ig + cytokines, and ii) effects of PC-conjugated tolerogenic antigens. These studies will define the importance of membrane Ig mediated antigen presentation and signal transduction in B cell activation and tolerance. 3. Antigen presentation by B cells in vivo The role of antigen-presenting B cells in initiating T cell responses in vivo will be studied in: i) SCID mice reconstituted with normal, recently immunized and memory T cells, with and without hapten-binding B cells, and ii) transgenic mice expressing the PC-specific membrane IgM, immunized with hapten-protein conjugates. 4. Mechanisms of B cell activation: cognate help and cytokines The responses of resting and activated B cells to Th-derived cytokines and to cognate interactions with Th clones specific for antigens r for class II MHC molecules will be studied, in order to define the mechanisms and contributions of these two types of T cell help in the induction of humoral immunity. Thus, this project will provide basic information about structure-function correlations of B cell membrane Ig, the cellular interactions in humoral immune responses, and the mechanisms of abnormalities in immunodeficient mice and in B cells producing functionally defective antigen receptors.